Mobile devices, for example mobile phones, typically include audio amplifiers which are intended to be mainly powered by battery. To achieve the desired audio output power level, a controlled voltage source may be used to generate the supply voltage of the audio amplifier. To improve the efficiency, the desired amplifier supply voltage may be increased only if the battery voltage is insufficient. This may be done by controlling the voltage source dependent on the amplitude level of the input audio signal. In many mobile devices, the controlled voltage source may be a DC-DC booster having capacitor at its output that is charged or discharged to vary the output voltage. The ramp-up time of the DC-DC booster is controlled to limit the peak-current drawn from the battery when the capacitor is charging up. Consequently, the DC-DC booster output reaches the target amplifier supply voltage somewhat later than required. When the DC-DC booster provides the required amplifier supply voltage later than required, the audio signal will be clipped resulting in audible distortion. The audio signal may be delayed to compensate for the DC-DC booster delay.